Twenty people were killed during the shooting and 26 people were injured when the gunman, identified by three sources as Patrick Crusius, a 21-year-old white man from Allen, Texas, began shooting just after 10:30 a.m.

"The state charge is capital murder, and so he is eligible for the death penalty. We will seek the death penalty," the district attorney said.

Authorities are investigating a post on the online message board 8chan believed to be from the suspect that laid out a dark vision of America overrun by Hispanic immigrants. The 2,300-word document, which police called a "manifesto," was attached to a post that said, "I'm probably going to die today."

The writing is filled with white nationalist language and racist hatred toward immigrants and Latinos. It blames immigrants and first-generation Americans for taking away jobs.

Although authorities are still investigating the suspect's connection to the document, Texas Gov. Greg Abbott called the shooting a "hate crime" during an evening news briefing, and the FBI has also opened a domestic terrorism investigation into the massacre, a source familiar with the investigation process said Saturday.

"This is disgusting, intolerable. It's not Texan," Abbott told reporters who asked about the document. "We are going to aggressively prosecute it both as capital murder, but also as a hate crime, which is what it appears to be, without having seen all the evidence yet."

FBI El Paso Special Agent in Charge Emmerson Buie said more investigative work was needed to determine whether there was a possible hate crime.